4.1 - 4.7 Energetics Flashcards
Define exothermic
Reaction gives out energy to surroundings (Temp increases)
Define endothermic
Reaction takes in energy from the surroundings (Temp decreases)
Define energetics/thermochemistry/thermodynamics
The study of heat changes during chemical reactions
Define temperature
- A measure of the average kinetic energy of the particles in a system
- Faster particles, greater kinetic energy, higher temp
Define heat
- A measure of the total energy of all the particles in a given amount of substance
Define enthalpy change (∆H)
A heat change at constant pressure
-∆H = Exothermic reaction
+∆H = Endothermic reaction
Define standard enthalpy change (ΔH°)
The heat change measured under standard conditions (100kPa and a stated temp, usually 298K) with all reactants and products in their standard states
Define standard enthalpy change of combustion (∆Hc°)
The enthalpy change when one mole of an element or compound is completely burned in oxygen, under standard condition, with all reactants and products in their standard states
Define standard enthalpy change of formation (∆Hf°)
The enthalpy change when one mole of an element or compound is formed from its element, under standard condition, with all reactants and products in their standard states
∆Hf°(any element) =
0
What is the equation for calculating heat energy transferred?
q = m x c x ∆T
q = Heat energy change (J)
m = Mass of water/solution
c = Specific heat capacity of water (4.18 J K⁻¹ g⁻¹ )
∆T = Temp change
What is the equation for converting heat energy transferred into enthalpy?
∆H = (q/1000)/moles
∆H = Enthalpy change (kJ mol⁻¹)
q = Heat energy change (J)
MUST MAKE IT NEGATIVE IF THE TEMP INCREASED
What is the name of the technique used to measure the enthalpy change of a reaction?
Calorimetry
Name the biggest source of error in calormietry and how to reduce it
- Loss of heat to the surroundings
- Reduce by = Insulation, bung callorimeter
State the 1st Law of Thermodynamics
Energy cannot be created or destroyed, only converted from one form to another
State Hess’s Law
The enthalpy change for a reaction is the same, whatever route is taken from reactants to products
∆H1 = ∆H2 + ∆H3
Which way do the arrows in an energy cycle point when formation is at the bottom?
Up
Which way do the arrows in an energy cycle point when combustion is at the bottom?
Down
Define bond dissociation enthalpy
The enthalpy (energy) required to break a particular covalent bond in a molecule when all species are in the gaseous state
Define mean bond enthalpy
The average value of the bond dissociation enthalpy (enthalpy to break a bond) in a particular type of covalent bond in a range of different compounds
Describe how to improve temperature recording for an enthalpy of reaction measurement
- Record the temp for a suitable time before adding the metal
- To establish an accurate initial temp
- Record temp at regular intervals of time
- Plot temp against time on a graph
Describe how to improve analysis of results for an enthalpy of reaction measurement
- Extrapolate the cooling back to the point of addition
- To establish a theoretical temp change
How to work out enthalpy change of a reaction when the bonds are given?
Bonds broken - Bonds formed
How do you calculate K1 from a graph of concentration against time?
Gradient
mol dm-3 s-1
